We're changing our Mac App Directory to suggest Sophos Anti-Virus for Mac as the best antivirus tool for OS X. Our previous recommendation was to run nothing at all, but there are simply too many robust and free options available to ignore. All of them protect your Mac from the malware that does target OS X, but the best ones also keep an eye out for an eliminate Windows viruses and malware so you don't inadvertantly infect Windows machines on your home network, a Windows partition you Boot Camp into regularly, or any Windows virtual machines that you keep on your Mac. Sophos does that and much more. You can find the full review here.